1. Field of the Invention
This invention relates generally to antifuses used in integrated circuit products and, in particular, to limiting the programming current used in conjunction with such antifuses.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Modern integrated circuit (I/C) manufacturing processes typically consist of numerous fabrications steps and produce chips which are simultaneously smaller and more complex. Each fabrication step introduces a new possibility for error which may render a die unusable. Examples of such errors include wafer imperfections or impurities, particulate or chemical contamination, insufficient etching, misalignment of masks, and scratches. Due to the reduced size and increased complexity of I/C devices, what were once acceptable imperfections may now render an I/C device unusable. Therefore, as the number of fabrication steps increases and the size of the product decreases, the actual yield of usable I/C products, measured as the percentage of faultless products, typically decreases.
In response to these potential decreases in yield, manufacturers have introduced higher levels of redundancy into the I/C devices being fabricated. As a result, a critical imperfection in one portion of the I/C device may be compensated for by a redundant component. Such redundancy increases the number of usable I/C products and therefore serves to increase the total yield. In the field of memory circuits, this redundancy has taken the form of providing extra columns and/or rows of memory cells which can be used to replace defective memory cells. In the event of memory cell defects, these extra columns or rows are activated and effectively replace the malfunctioning elements.
Implementation of this form of redundancy in memory circuits has relied upon structures known as antifuses. Specifically, in implementing memory circuit redundancy, antifuses are used as nonvolatile programmable memory elements which store logic states controlling the activation of redundant memory cell rows and columns. The antifuse functions as an open circuit until programmed. In addition to their functions in implementing memory cell redundancy, antifuses also may be used to change an operating mode or to encode identification information within an integrated circuit.
Structurally, an antifuse typically includes a polysilicon layer which forms the top plate of the antifuse element and which overlies an implanted region in the substrate forming the bottom plate. A dielectric exists between the two plates. The bottom plate of the antifuse element is usually contiguous with and comprises the source of the access transistor for the antifuse cell. When programmed, an antifuse creates a short circuit or low resistance link through the dielectric, thereby enabling the particular redundant row, column, or memory location.
A semiconductor die often contains a plurality of antifuse elements due to their use in programming redundant memory elements as well as their other uses. As dies have been scaled down, the density of memory elements on the die has increased. Ideally, the antifuses would be scaled down at the same rate as the rest of the die to avoid increasing the percentage of the die area devoted to antifuses. However, scaling down the antifuses has not been successful due to physical limitations of the antifuse structure. In particular, as the size of the antifuses has decreased, the programming current has not decreased at the same rate, leading to progressively greater current density. At these increased current densities, the junction of the bottom plate and the underlying substrate breaks down, creating an open circuit rather than the desired short circuit. The present invention may be directed to overcoming, or at least reducing the affects of, one or more of the problems set forth above.